Interrogation of the targeting mechanisms of ultrasound contrast agent microbubbles using Atomic Force Microscopy

The science of microbubble agents has expanded beyond imaging applications to biological targeting and drug/gene delivery. However, the majority of targeted microbubbles are manufactured without thorough characterisation of their targeting ability. Atomic Force Microscopy is capable of picoNewton force resolution, and is reported to measure single hydrogen bonds. The present study aims to introduce this nanosensor in the quantitative probing of the forces of interaction between cells and targeted microbubbles.

In-house, lipid based, targeted ultrasound contrast agents that use the biotin-avidin chemistry to carry CD31 antibodies probed cultures of SkHep1 cells with an Atomic Force Microscope. Tipless cantilevers were functionalised with poly-L-lysine and were immersed in a suspension of microbubbles in order to attach one at the end of each cantilever. This system then interrogated individual cells. In this initial study over 30 bubble-cell pairs were studied, producing over 200 force-distance curves. It was shown that the targeted microbubbles provide a significantly larger adhesion compared to control microbubbles. The average collective adhesion force was 0.68 +/- 0.33 nN and was dependent on the depth of contact. As the spatial and force resolution of the AFM is of subnanometer and subnanonewton level, respectively, it is possible to spatially resolve the adhesion sites of targeted microbubbles and measure the forces of these single sites. A histogram analysis of these data demonstrating a single distribution of adhesion events present in all measurements with median at 89.2 pN.

In conclusion, this system is capable of quantitative assessment of the avidity of targeted ultrasound contrast agents to cells, which is valuable information to the manufacturing process of such microbubbles.